worldcat.org
WorldCat is a union catalog that itemizes the collections of tens of thousands of institutions (mostly libraries), in many countries, that are current or past members of the OCLC global cooperative. It is operated by OCLC, Inc. Many of the OCLC member libraries collectively maintain WorldCat's database, the world's largest bibliographic database. The database includes other information sources in addition to member library collections. OCLC makes WorldCat itself available free to libraries, but the catalog is the foundation for other subscription OCLC services (such as resource sharing and collection management). WorldCat is used by librarians for cataloging and research and by the general public. More information...
According to PR-model, worldcat.org is ranked 5th in multilingual Wikipedia, in particular this website is ranked 5th in English Wikipedia.
The website is placed before archive.org and after nih.gov in the BestRef global ranking of the most important sources of Wikipedia.